Digital optical disks are now being seriously considered as potential alternatives to magnetic tape and disk memories. It has been found that optical disks offer a substantially greater data storage capacity than commercially available magnetic tape or disk memories of similar size.
The read/write head of many conventional optical disk systems generally includes a write beam optical path and a read beam optical path which is generally separate from the write beam optical path. The write beam optical path generally includes a relatively high power laser, some modulation scheme for the high power laser whereby an information modulated write beam is provided, beam conditioning optics, and mirrors which cause the write beam to illuminate one side of a beam splitter. The read beam optical path generally includes a lower power continuously emitting laser, beam conditioning optics, and mirrors which cause the read beam to illuminate the other side of the beam splitter. The write and read beams are combined by the beam splitter, with the combined beams first illuminating a polarization beam splitter, then illuminating a quarter (1/4) wavelength polarization plate, and subsequently being brought to focus on the optical disk by an objective lens. The read beam reflected off the disk is transmitted through the quarter (1/4) wavelength polarization plate to the polarization beam splitter which reflects the read beam into a second read beam path such that the read beam illuminates a detector which detects its intensity (to read data) and provides focusing and tracking information.
Reading recorded information immediately after writing that information, or direct read after write, would provide a number of system advantages. However, direct read after write is difficult to achieve reliably and economically with systems having separate laser sources because it is difficult to achieve relative positional stability between the write and read beams. Accordingly, there is a need for an effective direct read after write optical disk system which does not require alignment of two separate laser sources.